This invention concerns a procedure and a measuring cell for drilling fluids which make it possible to determine the adhesion and friction characteristics of the fluid during drilling.
It is known that, during the drilling of a geothermal or mining oil well, the physical-chemical equilibrium of the drilling fluid, termed the "drilling mud," undergoes constant modification because of changes in its constituents. The various load products (bentonite. polymers, etc.) undergo continuous violent shearing processes, both at the pumps and at the turbines or ventilation holes of the tool to these shearing stresses are added rises in temperature and pressure. Furthermore, the imperative maintenance of a fluid density sufficient to counterbalance the pore pressure of the formations being drilled dictates, additionally, that the soil loads will increase the concentration of the solid materials content or that the addition of barium sulfate will be purposefully made. In this content, the characteristics of the drilling fluid change substantially as the drilling continues, with the result that the preponderant coefficients change at the point of contact between the drilling wall and the drilling tool fitting, including the coefficient of adhesion to the fitting, the moment of adhesion to the fitting the moment of friction on the fitting, etc. The poor control of these characteristics leads to a deterioration of drilling performance: for example, through the poor transmission of weight over the tool, especially as regards the drilling of deviated or horizontal wells, and through an increased moment which hinders the proper rotation of the fitting and of the tool. It also leads to jamming of the fitting and to problems in the walls especially in cases in which clayey soils are being drilled. Clayey formations or layers often give rise, moreover, to a sudden increase in pressure, which must be overcome during drilling. This pressure is called "abnormal pressure" and is due to the addition of the pressure arising from the weight of the layers of underlying sediments to the hydrostatic pore pressure.